1. Field of the Invention
The present invention relates to a novel roughening method and a method for manufacturing a light-emitting diode having a roughened surface.
2. Description of Related Art
Currently, the research on GaN-based blue LEDs has been disclosed in international journals and technical reports with respect to photoelectric technologies. Their performance and manufacturing methods have been significantly improved. However, higher efficiency, more output power and enhanced luminous flux are required for the development of white light LEDs in the application of lighting and display fields.
FIG. 1A shows a conventional lateral GaN-based LED, which mainly includes: a sapphire substrate 11; a buffer layer 111 on the sapphire substrate 11; an epitaxy structure layer 12, including an n-GaN layer 121, an active layer 122 and a p-GaN layer 123 formed on the buffer layer 111 in sequence, therewith the n-GaN layer 121, the active layer 122 and the p-GaN layer 123 being partially removed to expose a partial area of the n-GaN layer 121; a transparent conductive layer 18 on the p-GaN layer 123; and two electrodes 13, 17, which are ohmic contacted with the p-GaN layer 123 and the n-GaN layer, respectively. However, the light output efficiency of the conventional lateral GaN-based LED in which the two electrodes 13, 17 are disposed at the same side of the epitaxy structure layer 12 is largely limited due to restricted effective area, long conduction path, large series resistance and current crowding effect near the electrodes. In particular, the light emission surface is close to the emissive layer (about 0.5 μm), and thus the surface roughening degree is limited. Consequently, the light output efficiency cannot be further enhanced, and conventional lateral LEDs cannot meet the requirements for high light output power. Additionally, in high power operation, the poor heat dissipation of the sapphire substrate 11 used in the conventional lateral structure would cause the reduction of luminous intensity and efficiency, and even the change of luminous wavelength and reduction of reliability and life, resulting in the serious limitation in high power operation.
To overcome those drawbacks of conventional lateral LEDs, vertical-structured GaN-based LEDs (abbreviated as VLEDs hereafter) have been suggested. In the VLEDs, two electrodes are respectively disposed at an upper side and a lower side of the epitaxy structure layer, and thus the thickness of the epitaxy structure layer is the distance between the two electrodes. Accordingly, the drawbacks of large series resistance in the conventional lateral LEDs can be resolved.
FIG. 1B shows a cross-sectional view of a vertical-structured GaN-based LED. As shown in FIG. 1B, the vertical-structured GaN-based LED mainly includes: a substitutive substrate 11′; an electrode 13, which includes an ohmic reflective layer 131 and an adhesive layer 132 and is disposed on the substitutive substrate 11′; an epitaxy structure layer 12, which includes a p-GaN layer 123, an active layer 122 and an n-GaN layer 121 on the electrode 13 in sequence; and an electrode 17, which is ohmic contacted with the n-GaN layer 121. In conventional art, the replacement of the sapphire substrate with the substitutive substrate 11′ is implemented with the use of laser-lift-off (LLO) technique and electroplated or wafer-bonded conductive substrates. In addition, the light emissionsurface of the VLED is far from the emissive layer, and thus the light output efficiency of VLEDs can be significantly enhanced by roughening the light emission surface. The suggested roughening technologies include: (1) creating photonic crystal on the surface by e-beam writer; (2) roughening the surface to form a textured structure of hexagonal cones by photo-assisted cryogenic etching; (3) performing a dry etching process with a metal nanomask (i.e. a photolithography technology); and (4) removing the u-GaN layer by inductively coupled plasma (ICP), and then performing a wet etching process to create a textured structure of hexagonal cones. Among them, the former three methods have the disadvantages of low throughput and difficulty in commercialization due to their complex process and long roughening time. Thereby, the fourth method is the commonly used roughening technology.